This invention relates to a fixture for use in measuring an electrical characteristic of a pogo pin.
In a semiconductor tester, stimulus and response signals are applied to and received from the device under test (DUT) by a test head. The DUT is mounted on a load board having contact pads connected to the pins of the DUT. The test head is equipped with spring-loaded contact elements known as pogo pins. A pogo pin is composed of a socket which is firmly secured in an insulating member, such as a plastic header or a circuit board, a barrel which is press fit into the socket, a plunger which is a sliding fit inside the barrel, and a spring inside the barrel and urging the plunger toward a projecting position. The load board is positioned so that the tips of the plungers are in contact with the conductive pads on the load board and the load board is then displaced toward the test head, establishing electrically conductive pressure contact between the tip of each plunger and the respective contact pad.
It is necessary for reliable and accurate operation of the tester that the electrical characteristics of the pogo pins, specifically the impedance of the pogo pins, lie within a fairly narrow range and be known with fairly high degree of accuracy.
Impedance has a frequency-independent resistive component and a frequency-dependent reactive component. Manufacturers of pogo pins conventionally specify only the resistance of a pogo pin.
In a high speed test of an integrated circuit device, frequency components of 500 MHz or higher may be involved, and in this case it may be desirable that the signal path between the test head and the DUT have a bandwidth greater than 1 GHz. The resistance value specified for a pogo pin is of no substantial relevance to the behavior of the pin at such high frequencies. For example, ideally there is a good sliding electrical contact between the plunger and the barrel but if there is instead a poor sliding electrical contact between the plunger and the barrel, and a significant part of the signal energy passes through the spring, the inductance of the spring may contribute a large reactive component of impedance that far outweighs the resistance of the pogo pin.
A time domain reflectometer (TDR) or a network analyzer can be used to measure high frequency electrical behavior of a conductive element. However, it is very difficult to couple the barrel or plunger of a pogo pin to a time domain reflectometer or network analyzer.